1. Field of the Invention
The present invention relates generally to managing signal interference on a printed circuit board and, more particularly, to interference signal decoupling using a board-level EMI shield that adheres to and conforms with printed circuit board component and board surfaces.
2. Related Art
As large scale integrated circuits operate at increasingly higher speeds, the need for devices to operate at faster switching rates has increased. As switching rates increase, problems which do not exist at lower speeds, such as shielding electronics from external sources of electromagnetic interference (EMI), become increasingly problematic. Moreover, maintaining sufficient electrical noise isolation and limiting inductance between components becomes more difficult as the frequency at which a circuit operates increases.
Conventionally, a shielded enclosure in the form of a metallic box or cage is placed around printed circuit board components to prevent unwanted electromagnetic energy from impinging on the protected components. Such metallic enclosures have numerous drawbacks that limit their shielding effectiveness. For example, electromagnetic energy often penetrates the on-board metallic enclosure at gaps between the cage and the printed circuit board surface. As a result, external electromagnetic fields can be capacitvely (electrostatically) coupled onto traces on the printed circuit board, magnetically coupled to conductive loops on the printed circuit board, or electromagnetically coupled to conductors acting as small antennas of electromagnetic radiation. In addition, power and signal lines are typically connected to interconnect posts mounted on the printed circuit board to receive power and transfer communication signals. Interfering signals conducted along such power and signal lines can enter the metallic enclosure.
In one aspect of the invention, a conformal coating noise suppressor for canceling interfering signals traveling on a conductor of a printed circuit board is disclosed. The noise suppressor comprises the conductor and a conductive coating of a conformal EMI shield conformingly adhered to surfaces of the printed circuit board. A portion of the conductive coating proximate to the conductor is configured such that the conductive coating and conductor form a first receiver loop in which signals induced therein travel through the conductor in a direction that is opposite the direction in which the interfering signals travel.
In another aspect of the invention, a decoupling circuit on a printed circuit board is disclosed. The decoupling circuit comprises an electrical filter circuit electrically connected between an interconnect post and a ground land of the printed circuit board. The ground land is connected to a ground plane in the printed circuit board. The electrical filter shunts conducted interfering signals received at the interconnect post to the ground plane. Components of the electrical filter circuit and ground plane form a first receiver loop in which induced interfering signals can be generated. The decoupling circuit also comprises a second receiver loop comprising a conductive coating of an EMI shield conformingly adhered to surfaces of the decoupling circuit, and a conductive path of the first receiver loop. The second receiver loop is adapted to have induced therein a signal having a direction and magnitude sufficient to cancel interfering signals induced in the first receiver loop.
In a further aspect of the invention, a printed circuit board is disclosed. The printed circuit board comprises a printed wiring board having a surface on which components are mounted. Also mounted on the printed wiring board is an interconnect post and a ground land connected to at least one ground plane. An electrical filter circuit comprising one or more components is also mounted on the printed wiring board. The electrical filter is electrically connected to the interconnect post and a first ground land. A conformal EMI shield is adhered to surfaces of the printed circuit board and comprises a dielectric coating directly contacting surfaces of the electrical filter components and adjacent surface traces. A conductive coating of the EMI shield directly contacts and covers the dielectric coating and first ground land.